Environmental risk due to micropollutants release: the contribution of wet-weather discharges in urban catchments

Wet-weather discharges from integrated urban wastewater systems (IUWS) pose a significant environmental risk to surface waters, particularly through Combined Sewer Overflows (CSOs) and Bypass (BP) discharges at wastewater treatment plants (WWTPs). The variability in micropollutant loads complicates risk assessment, yet recent revisions to the Urban Wastewater Treatment Directive emphasize the need for improved monitoring and impact evaluation. This study assesses the chronic environmental risk posed by 12 micropollutants in wet-weather discharges using an archetype IUWS, characterized stochastically through literature data. Six scenarios were explored, considering different micropollutant concentrations and river dilution factors to simulate climate change effects. Risk Quotients (RQ) were calculated to compare the contribution of CSOs, BP, and WWTP effluent. Results highlight the dominant role of dilution in risk assessment, with worst-case dilution scenarios amplifying risk by up to 10,000 times. Among micropollutants, Polycyclic Aromatic Hydrocarbons (PAHs) posed the highest risk, especially from CSOs and BP, followed by pharmaceuticals from WWTP effluent. This approach provides decision-makers with a robust framework for prioritizing mitigation strategies. However, site-specific monitoring campaigns and expanded micropollutant screening are necessary to refine risk assessments and address potential acute toxicity from wet-weather discharges.